Re: [PATCH 31/32] x86, pkeys: execute-only support

[Kees Cook]

On Mon, Dec 14, 2015 at 11:06 AM, Dave Hansen <dave@xxxxxxxx> wrote:
>
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>
> Protection keys provide new page-based protection in hardware.
> But, they have an interesting attribute: they only affect data
> accesses and never affect instruction fetches.  That means that
> if we set up some memory which is set as "access-disabled" via
> protection keys, we can still execute from it.
>
> This patch uses protection keys to set up mappings to do just that.
> If a user calls:
>
>         mmap(..., PROT_EXEC);
> or
>         mprotect(ptr, sz, PROT_EXEC);
>
> (note PROT_EXEC-only without PROT_READ/WRITE), the kernel will
> notice this, and set a special protection key on the memory.  It
> also sets the appropriate bits in the Protection Keys User Rights
> (PKRU) register so that the memory becomes unreadable and
> unwritable.
>
> I haven't found any userspace that does this today.

To realistically take advantage of this, it sounds like the linker
would need to know to keep bss and data page-aligned away from text,
and then set text to PROT_EXEC only?

Do you have any example linker scripts for this?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security
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